1. Field of the Invention
The present invention relates in general to optical loss measuring apparatus, and relates in particular to an apparatus for measuring connection loss and reflection attenuation in an optical cord provided with physical contact type connectors.
2. Description of the Related Art
A conventional apparatus for determining optical losses will be explained with reference to illustrations in FIG. 3. This is a case of measuring both the connection loss and reflection attenuation using the same apparatus.
As shown in FIG. 3, the optical loss measuring apparatus comprises a light source 1; an optical connector 2 having a semispherical end surface of high reflection attenuation, and is usually referred to as the physical contact (PC) connector; an optical coupler 3; a photo-detector 4; and a device fastener 6. An optical fiber cord (shortened to cord hereinbelow) 8 is the test cord to be measured, and the test cord 8 is provided at each end with first and second PC connectors 11 and 12, respectively.
To determine a connection loss of the cord 8 using the apparatus shown in FIG. 3A, a light of a specific optical power P.sub.0 dBm (0dBm=1mW) is emitted from the light source 1. The light is injected into a first PC connector 11 of the cord 8 through the optical coupler 3. As shown in FIG. 3B, the cord 8 is connected to the measuring apparatus by inserting the first PC connector 11 to the device PC connector 2 at the device fastener 6. The light propagates through the cord 8 and is emitted from the second PC connector 12 with an attenuated power P.sub.10 dBm. The connection loss .eta..sub.10 dB generated at the interface between the device PC connector 2 and the first PC connector 11 is obtained from the following equation. EQU .eta..sub.10 =P.sub.0 dBm-P.sub.10 dBm(dB) (1)
The determination of the reflection attenuation of a total reflection (fiber) cord 15 will be explained with reference to FIG. 3C. Light is injected into the cord 15 through a first (PC) connector 13 and propagates through the total reflection cord 15 to be reflected totally from the opposite end 16. The first connector 13 of the cord 15 is inserted in the measuring apparatus through the device connector 2 through the device fastener 6. Designating the connection loss of the injected light at the interface between the device connector 2 and the first connector 13 by .eta..sub.11 dB, the optical power P.sub.11 dBm of light injected into the total reflection cord 15 is given by the following equation. EQU P.sub.11 =P.sub.0 dBm-.eta..sub.11 dB(dBm) (2)
The injected light having an optical power Pucem is totally reflected at the opposite end 16 of the total reflection cord 15 at the same optical power P.sub.11 dBm. The return light passes through the device fastener 6 for the second time, (first time when the forward light passes through the device fastener 6) to generate a second connection loss of the same magnitude .eta..sub.11 dB. The optical power P.sub.21 dBm returning to the optical coupler 3 is given, from equation (2), as follows. ##EQU1##
The optical power P.sub.21 dBm of the return light, to be measured by the photo-detector 4, is attenuated when passing though the optical coupler 3 by an amount of connection loss 3dB. Therefore, the optical power reaching the photo-detector 4 is given by equation (3) as follows. ##EQU2## The value of the optical power P.sub.31 dBm given by equation (4) is the reference data for use in determining the reflection attenuation of the test cord 8.
Next, as shown in FIG. 3D, the total reflection cord 15 is replaced with the test cord 8 which is connected to the measuring apparatus through the first connector 11. The light emission section of the second connector 12 of the test cord 8 is coated with a refraction index adjusting coating 9 to suppress reflection therefrom nearly to zero. The first connector 11 of the test cord 8 is connected to the measuring apparatus at the device fastener 6 through the device connector 2.
Although the amount is minute, there is some reflection of the light forwarded from the light source 1 (forward light) within the device fastener 6 at the interface between the device connector 2 and the first connector 11. However, the reflection attenuation from the device connector 2 is so much greater than that from the first connector 11 that it can be neglected. Therefore, the amount of reflection generated at the interface can be attributed almost entirely to the first PC connector 11. Here, the reflection attenuation is expressed as a ratio (dB) of the optical powers of the forward light to the reflected light, therefore, the reflection attenuation R from the first connector 11 is given by the following equation. EQU R=P.sub.0 dBm-P.sub.42 dBm (5)
In the meanwhile, because of the action of the refraction index adjusting coating 9, the optical power P.sub.12 dBm of the forward light into the test cord 8 is almost all transmitted out into the space surrounding the chord 8. The reflected light is received by the photo-detector 4 through the optical coupler 3, but in passing through the coupler 3, P.sub.42 dBm suffers a propagation loss of 3dB. Therefore, the optical power reaching the photo-detector 4 is given by the following equation. EQU P.sub.52 =P.sub.42 dBm-3dB(dmn) (6)
In this equation (6), although the attenuation value R is expressed as a function of P.sub.0 dBm and P.sub.42 dBm, the quantity P.sub.42 dBm cannot be measured directly. Therefore, when the value of R is expressed as a function of measurable data using the relationships given in equations (4) and (6), the following equation is obtained. ##EQU3## Here, the third tern in equation (7) is small in comparison to the first and second terms and is also difficult to measure, therefore, the third term is generally neglected (ignored), and the expression is simplified as: EQU R=P.sub.31 dBm-P.sub.52 dBm(dB) (8)
R thus determined from equation (8) is usually taken as the value of the reflection attenuation of the first PC connector 11 of the test cord 8.
The conventional method for determining the connection loss and reflection attenuation presented above is based on connecting a total reflection fiber cord to obtain the reference intensity, but it is not possible to directly measure the connection loss introduced by connection to the optical coupler. The current state of the art is such that, because the reflection loss is small, it is neglected; however, the result is that currently it is not possible to determine the reflection attenuation precisely.
Another problem is that although a same measuring system is used, measurements must be carried out individually for connection loss and reflection attenuation. The result is that the measuring process is cumbersome and time-consuming.